Vertically aligned ZnO nanorod core-polypyrrole conducting polymer sheath and nanotube arrays for electrochemical supercapacitor energy storage
Nanoscale Research Letters
Vertically aligned ZnO nanorod core-polypyrrole conducting polymer sheath and nanotube arrays for electrochemical supercapacitor energy storage
Navjot Kaur Sidhu 0 1 2
Alok C Rastogi 0 1 2
0 Electrical and Computer Engineering Department and Center for Autonomous Solar Power (CASP), Binghamton University, State University of New York , New York 13902 , USA
1 Open PPy nanotube (4-h etch)
2 Authors' information NKS is presently a PhD student at the Electrical and Computer Engineering Department at the State University of New York, Binghamton. ACR is Associate Professor at the Electrical and Computer Engineering Department and Associate Director of the Center for Autonomous Solar Power (CASP) at the State University of New York , Binghamton
Nanocomposite electrodes having three-dimensional (3-D) nanoscale architecture comprising of vertically aligned ZnO nanorod array core-polypyrrole (PPy) conducting polymer sheath and the vertical PPy nanotube arrays have been investigated for supercapacitor energy storage. The electrodes in the ZnO nanorod core-PPy sheath structure are formed by preferential nucleation and deposition of PPy layer over hydrothermally synthesized vertical ZnO nanorod array by controlled pulsed current electropolymerization of pyrrole monomer under surfactant action. The vertical PPy nanotube arrays of different tube diameter are created by selective etching of the ZnO nanorod core in ammonia solution for different periods. Cyclic voltammetry studies show high areal-specific capacitance approximately 240 mF.cm−2 for open pore and approximately 180 mF.cm−2 for narrow 30-to-36-nm diameter PPy nanotube arrays attributed to intensive faradic processes arising from enhanced access of electrolyte ions through nanotube interior and exterior. Impedance spectroscopy studies show that capacitive response extends over larger frequency domain in electrodes with PPy nanotube structure. Simulation of Nyquist plots by electrical equivalent circuit modeling establishes that 3-D nanostructure is better represented by constant phase element which accounts for the inhomogeneous electrochemical redox processes. Charge-discharge studies at different current densities establish that kinetics of the redox process in PPy nanotube electrode is due to the limitation on electron transport rather than the diffusive process of electrolyte ions. The PPy nanotube electrodes show deep discharge capability with high coulomb efficiency and long-term charge-discharge cyclic studies show nondegrading performance of the specific areal capacitance tested for 5,000 cycles.
Zinc oxide nanorods; Polypyrrole nanotubes; 3-D nanostructures; Supercapacitor; Pulsed electrochemical polymerization; Electrochemical energy storage; Redox capacitance
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Background
Electrochemical energy storage in the ultracapacitor
devices is emerging as a frontline technology for
highpower applications ranging from modern portable
electronics to electric automotive. A battery-supercapacitor
hybrid energy system is a power source that can meet
the peak power demands in camera flashes, pulsed
lasers, and computer systems back-up as well as electric
propulsion in diverse industrial and vehicular transport
applications. Among the materials systems, structured
carbons which store charges as an electric double layer
(EDL) in liquid electrolyte medium are widely studied
with a focus on overcoming the energy-density
limitation [1]. The materials systems which show capacitive
function based on redox reactions are the insertion-type
metal oxides and doped-conducting polymers capable of
high energy-density storage [2,3]. The conducting
polymers, such as polypyrrole (PPy), poly(3,4
ethylenedioxythiophene) (PEDOT), and polyaniline (PANI) which
undergo redox processes equivalent of doping and
dedoping of electrolyte ions as means of energy storage
are being aggressively studied. These polymers exhibit
pseudocapacitance properties due to presence of charge
transfer reactions. The other most widely studied
materials are the metal oxides RuO2, MnO2, V2O5, NiO, and
Co3O4 which show highly capacitive behavior due to
reversible and fast surface redox reactions with
electrolyte ions [2,4].
In the recent years, conducting polymers with a
nanoporous morphology and as nanocomposites with
metaloxides have emerged as the materials system of great
potential for high energy-density storage. Electrodes
based on these materials structured at the nanoscale
enable many-fold enhancements of the electroactive
surface and interface with electrolyte facilitating absorption,
ingress, and diffusion of electrolyte ions which being the
main energy storage units could lead to increased energy
and power density of supercapacitor devices. The high
surface area morphology in conducting polymers is
attained by creating variations in its nanostructure like
nanoporous [5], nanofibers [6,7], nanowires [8],
nanobelts [9], and by size-selective nanopores in the context
of c (...truncated)